Multi-channel communication system



March `17, 1959 w. P. BooTHRoYD 2,878,316 MULTI-CHANNEL COMMUNICATION SYSTEM Original Filed Jan. 14, 1949` l14 Sheets-Sheet 1 M 3.* M on S s b March 17 1959 Y w. P. BooTHRoYD 2,878,316

MULTI-CHANNEL COMMUNICATION SYSTEM origina; Filed Jan. 14, 41949 Y 14 SheetsSheet 2 NQS kus Nn MNN .wt SSE JNVENToR umso/7 n 00m/roya V ,966/7 ro' @wmp/35M# Mardi 17 1959 w. P. BooTHoYD I 2,878,316 MULTI-CHANNEL COMMUNICATION SYSTEM Original FiledJan. 14, 1949 14 Sheets-Sheet 3 Fl .3g #or ra coma/mns c/n'cu/r .30 (F/GJ) mam ana ,I .sa Hua/0 f/Lm u 2z [1f/6.1) I n f l 1 J2 March 17, 1959 w. P. BoQTHRoYD 42,878,316 mmm-CHANNEL commmcmou SYSTEM original Filed Jan. 14. 1949 14`sheets-sneet 4 khas March 17, 1959 f w. P. BooTHRoYD 2,878,316

` MULTI-CHANNEL COMMUNICATION SYSTEM Original Filed Jan. 14, 1949 v y 14 Sheets-Sheet 5 10ML/eo /nPz/r puise: ren

IN VEN TOR lW. P. BOOTHROYD MULTI-CHANNEL COMMUNICATION SYSTEM March 17, 1959 14 Sheets-Sheet 7 Original Filed Jan. 14. 1949 March 1.7, 1959 w. P. BOQTHROYD MULTI-CHANNEL COMMUNICATION SYSTEM Original Filed Jan. 14',` 1949 14 Sheets-Sheet. 8i

,M IIIMIIIJMIIII n Ni NQQQRQQ W. P. BOOTHROYD n MULTI-CHANNEL COMMUNICATION SYSTEM Original Filed Jan. 14, 1949 `March 17, 1959 14 Sheets-Sheet 9 un* SEUR w. P. Boo'rHRoYD 2,878,316

MULTI-CHANNEL COMMUNICATION SYSTEM origina-1 Filed'Jan. 14, 1949 March 17, 1959 14 Sheets-Sheet 11 March 1 7,- 1959 w. P. Boo'rHRoYD MULTI-CHANNEL COMMUNICATION SYSTEM original Filed Jan. 14, 194s 14 snets-sneet 12 Y @una h IIIIIIIIQ'IIIIIIIIIIIIIII'IIIIIIIIIIIIII'IIIIII bww.

TS* ecs March 17, 1959 w. P. BooTHRoYD 2,878,316

*MULTI-*CHANNEL COMMUNICATION SYSTEM @M/5M 4MM March 17, 1959 W. P. BOOTHROYD 2,878,316

MULTI-CHANNEL COMMUNICATION SYSTEM United Wilson P. Boothroyd, Huntingdon Valley, Pa., assignorito 1 Philco Corporation, Phiiadelphia, Pa., a corporation of Pennsylvania Original application January 14,- 1.949, Serial No. 70,951, Vnow Patent No. 2,680,151, dated June 1, 1954. Di-

vided and this application May 3, 1954, Serial'iNo. 426,986

The present invention relates to multi-channel communication systems of the` pulse-amplitude-modulation type.

`The present application is a` division of myUnited States patent application, Serial No. 70,951, led January 14, 1949, which issued as U. S. Patent 2,680,151, on June 1, 1954.

Thepresent application also discloses subject matter which is described and claimed in a copending United States patentapplication` of E. M. Creamer,` Jr., Serial No. 70,952, tiled January 14, 1949, which issued as U. S. Patent 2,680,152, on June 1, 1954, as well as in a further copending United States patent application of` W. P. B'oothroyd andy E. M. Creamer, Jr., Serial No. 70,953, tiled January 14, 1949, which issuedA as U..S. Patent 2,680,153, on June 1, 1954.

` The two broad classes of multi-channel communication systems now in general useare (l) those in which the frequency `bands representing the individual signal `channels may occupy adjacent portions of a substantially continuous spectrum, and (2) those in which the entire frequency spectrum utilized is cyclically made available to each of the signal channels for a small time interval. The first of the above classes is commonly known as frequency-division multiplex, while the lattertis.,y similarly referred to as time-division multiplex.

The so-cailed frequency-division multiplex system Lof communication is widely employed, for example, where `a number of telephone conversations or telegraph messages are to be carried over a single cable. It is alsosuitable for use where such signals are `to be transmitted by certain types of radio relay'networks. In this latterapplication, it has the advantage `of operatingwith a desirably loul bandwidth. At the same time, it possesses the disadvantage in its present form of being unable to load many other types of relay equipments to their maximum capacity. This is especially true where these relays are designed primarily for the transmission of television yor other Wide-band signals.

'lhe time-division multiplex system` of communication is actualiy a method of pulse transmission` and may be sub-classified in accordance with the manner in which the pulses are modulated. These sub-classes` generally include (a) varying the pulse amplitude, and (b) varying the pulse position, that is, changing the time of Occurrence ot either the ieading or trailing edge `ot the pulse, or both. The pulse-amplitude-moduiation method,'in which the various intelligence signal channels differ from one another in amplitude, has been frequently. employed. and possesses the advantage Vof operating with adequate linearity. Heretofore, however, the excessively high bandwidth required was a factor in limiting` the extent of its use. The pulse-position-modulation method, on the other hand, has the advantage of being relatively unaffected by attenuation in the transmission path,;but` requiresrawider frequency band for successful operation.

It is desirable incertain typesiof,multifchanneliconb munication systems, especially those ,making 115e of Patent f ...cables Aor relatively small .radiorelayi stationsyto' have :mum bandwidth is required.` However, thissame relay apparatus should at the same `time be usablewith other modulators whichV are primarily designed for television,

FM programs, andl other high-frequency applications. `In other words, theterminal equipment should `be capabley of use without major alterations in conjunction with either type o f modulating apparatus.

i Although one of the principali features-ofy theiifrequencydivision-multiplexing method is its relatively narrow bandwidth, neverthelessit has-been found that an amplitudemodulated-multiplexing system can bel devisedwhich not only equals the frequency-division method with respectto bandwidth economy, butvwhich `in` addition possesses adequate linearity response. This system, to be-later described; `operates y over a portiontof'theV spectrum which is a practicalwequivalent of that required by a single-sideband frequency-division-multiplexing system, andat the same time presents--nolinearity problems when theamplitude-modulated.multiplexedsignalsis employed to' frequency-modulate a carrier wave `for transmission. It has also been foundthat such an amplitude-modulatedsystem provides :an `adequate signalto-noise' ratio,I .and substantially minimizescrosstalk between channels incomparison with` other time-division-multiplex` systems employing pulse modulation.

One-of the operatingiprinciples of pulse `communication systems is that if.an intelligence signal beisampled Aat regular intervals, the resulting signal will still retain substantially all 4olthe, useful Ainformation present in` the original signabprovided fthat.the sampling frequency is at a rate equal to at leastA twicethe highestiuseful frequency in the `original signal. Iny other words, the intelligence may be reproduced substantallyin its original form if the sampling period is equal to approximately one-half the period ofthe highest frequency-component of the originalwave. For example,.in thecasetotJ an audio wave which has been passedthrough a filter having a cut-off frequency of approximately 3,500 cycles, then substantially all of theaudio information in the wave is present in a series` of, samples ofthe wave taken at anv 8 kilocycle rate.

This sampling principle hasbeen utilizediin designing the pulse-amplitude-modulated time-sharing-multiplex vsystem described in the, presentapplication. In one-embodiment of the invention, Vsamples from a. plurality of audio-frequency channels arecombined into an asymmetrical composite signal comprising a train of` amplitudemodulated pulses. This system is arranged not only to give adequate linearity, but in addition to be readily adaptable for use either in connectionwith high-frequency relaying apparatus or with relay networks employing pulse transmission only. `Furthermorethe multiplexed signal may frequency-modulate a carrier wave directly, or may modulate a sub-carrier wave.

In a physical embodiment `of the `system to be `described, thirty separate and independent audio frequency channels are time-multiplexed into a 150 kilocycle fre quency band. One of these channels transmitsanindexing tone for synchronizing purposes, and is also used as an order line. The remaining twenty-nine channels are `available for any'desired form of audible communication, such as ordinary telephone conversation, or for telegraphy.

Each audiochannel is `designed with a frequency pass band ofbetween 300 Iand.3,'300 cycles per second, and thus has an audio delity' corresponding to that of a typical telephone system. MThe passband of thev order line is from `300 to 2,500 cyclesrper second,zwvith the indexing (or synchronizing) signal occupying a portion ofl the remaining .space in` thischannel. The` thirtyaudio signals respece tively occupying the thirty audio frequency channels are combined into a pulse-amplitude-modulated time-multiplexed composite signal, which may then be applied either to modulate the carrier wave of a transmitter, or else sent out directly over a single cable. At the receiver, the composite multiplexed signal is resolved into its audiofrequency components. Inasmuch as the thirty channel system requires a bandwidth of only approximately l() kilocycles, or less, it compares favorably in this respect with any other known system of multi-channel communication of either the frequency-division or the time-division species.

One object of the present invention, therefore, is to provide an improved intelligence-communication system of the Ypulse-amplitude-modulation type.

Another object of the invention is to provide a communication system of the pulse-amplitude-modulation type in which the frequency band required for transmission is approximately equal to the normal spectrum of the and in which crosstalk between channels is reduced to a minimum.

Other objects and features of the invention will be apparent from the following description of a preferred embodiment and from the drawings, in which:

Fig. 1 is a block diagram of a preferred form of multiplex communication transmitter system in accordance with the present invention;

Fig. 2 is a block diagram of a preferred form of multiplex receiving system in accordance with the present invention;

Fig. 3a is a circuit diagram of one form of modulator included in the transmitting system of Fig. l;

Fig. 3b is a set of waveforms helpful in explaining the operation of the modulator of Fig. 3a;

Fig. 3c illustrates the general characteristics of one form of band-limiting network in Fig. l;

Fig. 3d illustrates the circuit of a preferred type of lter which is adapted to perform the function of the bandlimiting network of Fig. 3c;

Fig. 3e is a graph of loss vs. frequency for one particular type of such a band-limiting network;

Fig. 4 is a set of idealized waveforms which are helpful in explaining the operation of the filter of Fig. 3d;

Fig. S illustrates the circuit details of one form of correction network included in the receiving system of Fig. 2;

Fig. 6 illustrates graphically in an idealized manner the operation of the crosstalk-correcting network of Fig. 5;

Fig. 7 is a block diagram of the timing generator included in the receiver of Fig. 2;

Fig. 8 shows the circuit components of Fig. 7;

Fig. 9 is a set of waveforms which are helpful in explaining the operation of the system of Fig. 8;

Fig. 10 is a block diagram of two of the channel separators included in the receiver of Fig. 2;

Fig. 11 illustrates the circuit components of Fig. 10;

Fig. 12a illustrates the response characteristic of another band-limiting network of the type shown in Fig. 3c;

Fig. 12b illustrates the approximate relative amounts of crosstalk present in adjacent channels when employing 4 voltages, at time intervals equal to the channel intervals, for a lter having the characteristics of Fig. 3e;

Fig. 14b is a block diagram of a corrector network for substantially eliminating the crosstalk shown in Fig. 14a;

Fig. 14cis a table of output voltages from the corrector network of Fig. 14h at the time intervals shown in Fig. 14a; and

Figs. 15a and 15b are graphs of amplitude vs. frequency and amplitude vs. time, respectively, for a modied form of the filter network of Fig. 3d.

In accordance with a principal feature of the present invention, each intelligence channel is sampled at a rate dependent upon the highest intelligence frequency contained in the signal in that channel. This sampling process detects the instantaneous amplitude of the signal in cach channel, and, by sampling the channels in sequence, the channel information is made available for interleaving into a composite multiplexed signal.

In a preferred embodiment, the apparatus for carrying out the above process includes a pulse generator feeding an artificial delay line. The pulse from the generator is preferably of triangular waveform, and has a recurrence frequency equal to the desired sampling frequency, which may for example be 8 kilocycles. The delay line is provided with a plurality of output terminals equal in number to the number of intelligence signal channels. In the embodiment being described, the delay line is designed with thirty equally-spaced taps giving approximately a 4.16 microsecond delay therebetween. A lowpass lter is incorporated in the input section of the delay line, this filter acting to remove the high-harmonic components of the triangular input pulses. At all of the output terminals of the delay line, therefore, there are available properly-timed sampling waves of the same shape and containing substantially no high-frequency energy. One type of apparatus particularly suited for producing a sampling wave of the above nature is described and claimed in applicants copending application Serial No. 14,691, filed March 13, 1948, which issued as United States Patent 2,616,047 on October 28, 1952.

As described above, the delay line input pulse, passing the various output taps in sequence, becomes a timing wave for the purpose of sampling the respective intelligence channels. This sampling is preferably accomplished by gating a normally non-conductive electron tube incorporated in each intelligence channel of the system. The intelligence signal in each channel is continuously applied to its respective gating tube, and the latter is rendered conductive only upon the application thereto of one particular timing or gating pulse from the delay line. The output of each gating tube is consequently a signal having an amplitude representative of the amplitude of the intelligence signal at the instant when sampling occurs. i

The respective outputs of these gating tubes are combined in timed sequence to form a composite multiplexed signal which may then be transmitted directly or employed to either amplitude-modulate or frequency-modulate a carrier wave for transmission by any suitable form of translating device.

It has been found that the bandwidth necessary for the satisfactory reproduction of the intelligence contained iu each of the signal channels may be minimized without introducing excessive crosstalk or distortion in the reproduced signal by employing, just before the transmitter, a filter having a relatively low cut-off. The underlying principle in this arrangement is that it is unnecessary to transmit all of the harmonics and all of the sidebands present in the combined output of the gating devices. For example, in the illustration given in which each channel of a thirty channel system is sampled at an 8 kilocycle rate, substantially all of the intelligence may be retained in the signal if the 8 kilocycle sampling wave is transmitted with `its harmonics (up to about the fgarants teenth), each of which is modulated with a group of sidebands `extending about 3.3 `kilocycles to each side thereof (assumingthat the highest intelligence signal frequency `is 3,300 cycles). It has further been found that distortion may be almosttcompletely avoided if both sidebands of each `useful harmonic of this 8 kilocycle'wave are passed withtsubstantially equal amplitude. i

Accordingly, the output iilter is designed to cut off at approximately 150 kilocycles, the response being fairly uniform up `to this point with a rapid attenuation thereafterso as to be down db at 260 kilocycles. Such an attenuation of the higher frequencies, however, causes a widening, or spreading, of the signal energy in each channel, and hence the system includes means for overcoming `the diiiculties arising from this condition.

Inasmuchasthe present disclosure employs amplitude` modulation, itis necessary that at the relay receiver the received `signal be sampled at each precise instant when its `amplitude is representative of the intelligence being conveyed. Furthermore, no cross-modulation between channels is permissible at the time when sampling occurs. In `order to insure that Vthese conditions prevail, means are provided for reducing substantially to zero the energy in'the adjacent signal channels at the instant when the sampling of any one particular channel takes place.

The means for accomplishing this result include in a preferred embodiment a so-called correction network to which the multiplexed signal at the receiver is applied. By properly adjusting both the time delay and the phase displacement `of each particular portion of the multiplexed wave by the use of this correcting network, it is possible tosubstantially cancel any energy resulting from signals previously transmitted and remaining in any one portion of the wave at the `time when the wave portion representing the next succeeding intelligence channel appears at the input terminals of the network. Hence, no carry-over or residual energy remains to cause distortion of the intelligence signal, or crosstalk.

It has also been found that an unmodulated reference wave, having a frequency equal to the sampling frequency multiplied by the number of intelligence channels, may be used at the receiver to provide a reference level, or base, on which the amplitude-modulated intelligence signal may be superimposed. This permits the demodulation of the signal to be more readily accomplished without crosstalk. It has also been found that this reference wave may be derived by transmitting a sub-harmonic f the reference frequency (falling within the passband of the output iilter system) and then frequency-multiply ing the received sub-harmonic signal to obtain the desired wave.

lt will thus be recognized that the present disclosure provides a pulse-amplitude-modulated multichannel communcation system which employs a time-multiplexed signal derived by sampling in sequence a plurality of intelligence signals, the multiplexed signal being modulated in amplitude in accordance with the intelligence signal level at the time of sampling. It will be further noted that the transmitted signal contains no frequency higher than that ordinarily present in an equivalent single-side# band frequency-division system.

Transmitter claimed in the copending application Serial No. 14,691, `referred tc above.

The repetition rate of the pulses produced by the generator 10 is 8 kilocycles. In other words, the peak f each pulse isspaced in time from the peak of the immediately preceding pulse by an interval of microseconds. -liurthermore, for reasons which will later become apparent, each triangular pulse has :an effective width at its base which is no greater than 8 microseconds.v

These pulses from the generator 10, which may have a waveformsuch as shown in the drawing by the reference numeral 12, are applied tothe input terminal of a delay network 1d. This network 14 may be of any forrnknown in the art, such, for example, as a plurality of series-connected inductors and shunt-connected capacitors arranged to form individual sections or units. Network 14 isprovided with 30 equally-spaced output taps chosen so that the time delay for each section-of the network is approximately `4.16 microseconds. The total delay interval for the entire network is thus 4.16, 30, or 125 microseconds, and is substantially equal to the period of the pulses l2. Such delay networks are known in the art, but one type of delaynetwork which is particularly suited for `this purpose is `shown in the copending application Serial No. 14,691.

In order that the `delay network 14 may remove the high-frequency components present in the pulse output of the generator 10, a low-pass filter is incorporated therein. This iilter is designed to have a cut-off frequency of, lfor example, 200 kilocycles. It may take the form of a number of extra L-C sections located at the input end of the delay network. Accordingly, the pulses 12, which appear successively at the output taps #l-#BO of the network 14, have a waveform in which the sharp peak ofeach pulse is 'rounded od, as shown by the reference numeral 16.

The fcharacteristic impedance of the delay network 14 is of course determined by the particular values of theindoctors and capacitors making up the assembly. It has been found `in practice that a characteristic impedance of 2500 ohms will produce satisfactory results, and a suitable terminating impedance of this value is used.

One important `characteristic of the delay network 14 is that it doesnot introduce any appreciable change in the waveform of the pulses 16 as they travel therealong. After the output pulses 12 from the generator 10 have passed through the rst few sections of the delay network 14 (which constitute the low-pass filter), and have arrived at the first output tap of the network with the shape shown at 16, no signiiicant change occurs in the waveform of the pulses until after they pass the last output terminal #30.

Summarizing the above, a pulse 12 applied to the input terminal of the network 14 appears at the output taps #1-#30 with `the waveform 16 successively at times spaced approximately 4.16 microseconds apart. The wave retains substantially this same shape at each output terminal of the network.

As previously mentioned, the transmitting system of Fig. 1 is designed to multiplex thirty audio channels on a time-sharingbasis. This is accomplished by sampling the intelligence signal `in each channel at a rate equal to at least twice the highest frequency contained therein. This sampling process detects the instantaneous amplitude of the intelligence signal at the instant when sampling occurs, and, since the channels are sampled in sequence, the channel information is available for intermixing into a composite multiplexed signal.

As the pulse 16 passes the various output taps ofthe network 14 in sequence, it becomes a timing wave for the purpose of sampling the respective intelligence channels. The embodiment of the invention illustrated includes thirty such channels, although this number was arbitrarily chosen and thus is merely exemplary. One` of these channels `transmits an indexing tone for synchroniz-I ing purposes and is also used as an order line.. The re# maining twenty-nine channels are available for audio communication.

'arogare Each of the thirty output taps or terminals of the delay network 14 is connected to one of thirty modulators 18. Twenty-nine of these modulators also receive signals from twenty-nine audio input channels, each of which includes a microphone 20 or other source of audio frequency signals. In order that all frequencies outside the 300 to 3,300 cycle range may be eliminated from the output of the microphones 20, a filter 22 is provided in each audio channel. The output of each filter 22, therefore, is an audio signal having no frequency higher than approximately 3,300 cycles per second.

Each of these audio signals is applied to its respective modulator 18, which also receives a timing signal, in the manner above described, from one output tap on the delay network 14. Inasmuch as the highest audio frequency is limited by the filters 22 to a value of approximately 3,300 cycles per second, it will be seen that the 8 kilocycle wave 16 will sample the audio information in each channel at a rate equal to at least twice the highest audio frequency. Furthermore, the amplitude of the 8 kilocycle energy appearing at the output of any particular one of the modulators 18 will depend upon the instantaneous value of the audio signal applied to that particular modulator at the instant when a sampling pulse is also applied thereto. Thus the signal in each audio channel may be transmitted without any appreciable loss of the inforrnation contained therein.

The single remaining modulator #27 receives both an indexing tone at a frequency of 3,900 cycles from a generator 24 and also the output of an order line filter 26, Inasmuch as the order line information in the embodiment described does not require as high a frequency range as that of the remaining audio inputs, the order line filter 26 (which is connected to a microphone 28), has a frequency passband of from 300 to 2,500 cycles. Since the highest frequency applied to the indexing tone and orderl line modulator is 3,900 cycles per second, the intelligence in channel #27 will still be sampled at least twice per cycle by the 8 kilocycle timing wave 16.

Although any one of the modulators 18 might have been selected to receive the combined output of the indexing tone generator 24 and the order line filter 26, in the present embodiment channel #27 was selected for this purpose. Thus, each one of the thirty modulators 18 is connected to receive a triggering pulse in timed sequence from one of the thirty taps on the delay network 14.

The respective outputs of the modulators 18, represent ing thirty channels of amplitude-modulated pulses, are then combined into a single multiplexed signal by the combining circuit 30. The signal in the output of the circuit 30 may have a waveform such as represented by the reference numeral 32. This wave 32 is a composite multiplexed signal composed of thirty phase-delayed pulses derived from each one of the 8 kilocycle timing pulses 16, or 240,000 amplitude-modulated pulses per second. Each thirtieth pulse in this wave represents the intelligence of one particular channel.

One of the principal features of the present invention resides in the ability of the disclosed apparatus to operate with a bandwidth which is approximately equal to the normal spectrum of the multiplexed signal. It has been found that the transmitted intelligence will be reproduced with negligible distortion if the 8 kilocycle sampling wave is transmitted with a substantial number of its harmonics, each of these harmonics having sidebands extending a dis* tance on each side thereof equal approximately to the highest channel frequency. It has furthermore been found that an unmodulated reference wave representing the 240 kilocycle sampling frequency is desirable at the receiver in order to aid in the demodulation of the multiplexed sig nal.

According to this feature of the present invention therefore, a transmission bandwith of only approximately 1 50, kilocycles is required. Although this permits transmisv. and applied to a suitable transducer so that the intelligence contained in the signal maybe reproduced.

sion of an appreciable number of the harmonics in the 8 kilocycle sampling wave, it does not permit transmission of the unmodulated 240 kilocycle wave. However, if a subharmonic of this 240 kilocycle frequency is derived at the transmitter (such as kilocycles, for example), it may be added to the transmitted signal and then restored to its original form at the receiver by utilizing a suitable frequency-doubling circuit. 1

In order that the multiplexed signal may be transmitted within the above-mentioned kilocycle passband, the composite signal 32 is applied to a band-limiting network 34 which in effect consists of a low-pass filter having a response which drops only gradually up to 150 kilocycles but then falls off sharply until it is down substantially 40 db at 260 kilocycles. This band-limiting apparatus permits transmission of the 8 kilocyclesampling'wave with a number of its harmonics (up to at least the fifteenth), and, furthermore, passes the two sidebands of each of these harmonics with substantially equal amplitude. However, it is recognized that the cut-off of the band-limiting network 34, such as shown by the response curve 36 in Fig. l, will introduce considerable crosstalk into the signal 32 unless it is compensated for. The means for producing such a compensation are an essential portion of the invention, and will be fully described in connection with a description of the receiving apparatus, as set forth below.

A. portion of the output of the oscillator 8 is applied to a 120 kilocycle filter 38, which may also, if necessary, include suitable clipping and amplifying means for producing an output wave 39 of constant amplitude. YAny necessary phasing of the wave 39 may be brought about by a phasing unit 40. The output of the unit 40 is come bined in properly timed relation with the output of the band-limiting network 34, and the resulting wave is employed to modulate either a transmitter 42 or any other type of translating device. However, it should be understood that the wave representing the combined outputs of the band-limiting network 34 and the phasing unit 40 may also be transmitted yby a cable or other form of wire-transmission medium.

Receiver v In Fig. 2 is illustrated a block diagram of one form of multiplex receiving system in accordance with the present invention. The receiving system of Fig. 2 is particularly suited to reproduce the intelligence present in a multiplexed signal transmitted by a system such as illustrated in Fig. l. l

Broadly set forth, the incoming signal is detected byl a receiver and applied to a network which is effective toy correct or compensate for the spreading of the signal channel energy introduced by the band-limiting circuit of the transmitter. A 120 kilocycle wave is also derivedy from the received signal, doubled in frequency, phased, and combined with the signal output of the correcting network. This combined signal is then amplified and applied to each of thirty channel separators which also receive gating pulses from a timing generator the operation of which is indexed with the operation of the timing generator at the transmitter by means of a control voltage derived from the energy in the indexing tone channel. The signal output of each channel separator is filtered Referring now to the particular elements of Fig. 2, the signal is first detected by a receiver 50 which may be ofconventional design. The output of this receiver 50 may be a series of amplitude-modulated pulses having a wave*4 form similar to that representing the combined outputsx 9 ously mentioned, considerable spreading of the signal channel energies is introduced into the transmitted `.signal `incorporated in the :limiting network 34. Consequently, the receiver of uFig. 2 includes a correction network 52 to which `the output of the` receiver= 50 `is applied.

Thellter network 34 or" Fig. 1 preferably has a response characteristic which, while sloping only slightly out .to a .frequency of approximately 150 kilocycles,

,nevertheless is not completely flat over this @portion of the spectrum. Although it may be down only approximately `8 dhr-at 150 `kilocycles, even this relatively slight slope .is enough to produce crosstalk between adjacent vintelligence channels. `If some :means were not present vin `thesystem to reduce this distortion, it would be extremely diicult to reproduce satisfactorily the audio `.signals .or other linformation imparted to the system at the transmitter. However, by means of the correction `network 52, any such crosstalk which may exist is reduced -lto ra `negligible value.

lIn 4one form, `the correction network `52 includes first an artificial delayline which is terminated in other than `its characteristic impedance. lt therefore `produces reilections, the phase and amplitude of which can heiselec- `.tively controlled. In other words, the reflection produced by the appearance of a pulse representing one :particular signal channel at the input of the delay line may be so controlled in amplitude and phase as to arrive back at the -input terminals of the line with a potential which is equal andopposite to the residual voltage of that particular ,pulse remaining at `the input terminals at the 4precise `instant of arrival of the pulse representing the next succeeding channel. Hence, the only signal electively vpresent at the time of arrival of a following pulse is that which is actually present in the latter pulse itself, `and no residual or carry-over voltage remains from the pulse which `preceded it. A second delay line is employed to compensate for crosstalk introduced .from the energy in the immediately following channel. A complete description of the details of the correction network 52 will be given in connection with a description of Fig. 5, and it is believed that the above is sufficient at this 'point to provide an understanding of the function f this particular component in the receiver system.

It will be appreciated from the above description that the correction network 52 acts to reduce crosstalk between adjacent channels at one `precise instant in each cycle when the residual voltage of a particular pulse is cancelled by the presence of the equal and opposite voltage derived by rellection. However, it will also be clear that foreach channel this cancellation occurs at only one instant. "Hence, in order to derive a signal representative of the lactual lintelligence present in the received Wave, itisnecessary that the various channels he sampled at the exact moments when such crosstalk cancellations occur. It isfor this purpose that the `l2() kilocycle wave output of `the phasing unit lll in Fig. 1 was combined withthe output of the hand-limiting network 34.

Referring again to Fig. 2, there is provided a filter 54 which is connected Las shown `to the receiver 50 so as to produce a 12h kilocycle energy wave bearing a timed relation to the received signal. This 120 kilocycle energy from filter ft, which is unmodulated, is then passed through a frequency doubler 56 and a phasing unit 58 to produce a 240 kilocycle wave of constant amplitude, part of which `is mixed with the intelligence signal output of the corrector network 52 in an amplifier and cathode follower 59. The phasing unit 58 should be adjusted `so that the peaks of the 240 kilocycle wave occur at the precise instants when the correction net- Work 52 yreduces the crosstalk between the adjacent intelligence channels substantially to zero. In other words, units 54, 56 and 58 act to provide a hase, or pedestal, upon which the amplitude-modulated multi plexed "signal output of the correction network 52 may l0 be superimposed. This 'multiplexed signal, which now `possesses a reference `or fbase voltage, -is applied simultaneously over the conductor 60 to each one of thirty channel separators 62.

The `receiver of `Fig 2 also includes a timing generator 64 `which has `functions similar to that of the pulse generator 1t) in combination with the fdelay network 14 in Pig. 1. rlT-hat is, the .generator `64 provides a `timing wave which `is produced by pulses of an E kilocycle repetition `frequency traversing a delay line having thirty equally-spaced output taps. The delay period between the successive output taps is identical to that provided by the delay network 14 in Fig. 1 or, in other words, about 4.16 microseconds. The details of `this timing generator 64 will be set forth in connection with a description of Figs. 7, `8 and 9, and it will merely be stated at this time that Vthe generator 6d receives both a synchronizing voltage from an indexing tone lilter 66 over a conductor` 67, and also a portion of the unmodulated 240 kilocycle output `of the phasing unit 58 over a conductor 68.

The `thirty channel separators 62, including the indexing tone :and order line vchannel separator #27, are all supplied with the intelligence signal from the amplifier- 59, and also with timing pulses from the generator 64. These latter pulses `gate the channel separators 62 in such a manner that there is no output from the latter except during the occurrence of a timing pulse. However, `when such a timing pulse does occur, then the output voltage from the particular separator 62 to which it is applied rises to a peak value corresponding to the amplitude of the pulse included in the intelligence signal at `the instant of triggering.

Each of the channel separators 62 thus in effect selects one particular channel lfrom the composite multiplexed signal. In order that sufficient power be available which is truly representative of the intelligence in that particular signal channel, it is desirable that the output wave from each separator be maintained at the intelligence signal level for a sucient period of time to provide adequate energy for the reproducing apparatus. Accordingly, each of the channel separators 62 is so arranged that the timing pulse from the generator 64 acts t0 initiate a voltage variation which remains at a constant level `for an appreciable period of time, and is then returned to its original value by an action of a-discharge,` or restoring, voltage derived from the immediately preceding channel. In the embodiment illustrated, the voltage `in each of the channel separators which is representative of the intelligence information is caused to remain constant for a time interval of about 12() microseconds,1thisbeing slightly less than the mi-crosecond period of the 8 kilocycle timing pulses. `Further details of the channel separators 62 will be given in connection with a description of Figs. l0 `and `111.

The output of` channel separator #27 `is applied not only `to lthe indexing tone lter 66 to permit `separation of the `3,900 cycle synchronizing wave, but also to a low pass `(300 to v2,500 cycle) filter 7d to provide the order line intelligence picked up `by the microphone 28 at the transmitter. The remaining twenty-nine channel separators `are 4connected to twentynine audio lters 72, the respective outputs of which are reproduced in the twenty-nine `output circuits thereof, here represented by twenty-nine audio reproducers 74.

Modulators In Fig. .3a is shown `a preferred `type of circuit 4for accomplishing the function olf-each individual modulator 'i8 in Fig. 1. "t `will be appreciated that it is the purpose of each suchmodulator 18 to act as a gating `circuit which effectively connects the output of its respective audio `lter `22 to the modulator `output cir-cuit (combin ingcircuit 30.) -in accordance with the application -Ato the modulator of one of the timing pulses 16` trom the ft- 11 delay network 14. Theoutputs of the respective modus lators are then consolidated as shown in Fig. 1 in the combining circuit 30. It will be further appreciated that the modulator 18, or gating circuit, must be closed to the output of its associated audio ilter 22 at all times except when it is opened by the application thereto of one of the timing pulses 16 from the delay network.

Accordingly, each one of the modulators 18 in Fig. l may include a pentode 76 (as shown in Fig. 3a) to the control grid 78 of which the output of an audio filter 22 is applied. The screen grid 80 of tube 76 is connected to a source of positive potential through a resistor 82. The cathode 84 of the tube is connectdd to ground through two series resistors 86a and 86h, and is also connected directly to the anode of a diode 88. The cathode of diode 88 is connected to a source of negative potential through the resistor 89.

The pulses 16 from the delay network 14 are applied with postiive polarity to the suppressor grid 90 of the pentode 76 by means of conductor 91 and condenser 92. The bias on suppressor grid 90 is maintained at a selected negative potential in excess of cut oi by having its leak resistor 92a connected to the arm of the potentiometer 93 which is connected between a negative potential source and ground. The output of the pentode 76 is taken directly from its plate 94, and is applied as shown to the combining circuit (Fig. l).

The audio voltage from the filter 22 in Fig. 1 is accordingly applied to one grid of the pentode 76, while the timing pulses 16 from the delay network 14 are applied to another grid of the same tube. In this manner, the pentode is gated open by the timing pulses so as to pass into its anode circuit a voltage representative of the instantaneous amplitude of the audio signal during the presence of the gating pulse. However, it has been found that the amplitude of the output signals from anode 94 will not be linearly related to the applied audio signal unless the gating voltage on its suppressor grid 92 can be made to vary as a function of variations in the audio voltage appearing on its control grid '78 and hence on its cathode 84.

The means for bringing about such a correlation in the applied tube potentials includes the diode S8. Since the voltage on the anode of this diode is substantially the same as the voltage on its cathode under normal operating conditions, because the diode is normally conducting, it will be seen that the alternating component of its anode voltage (which is the voltage on the cathode 84 of the pentode 76) will traverse condenser 99 and appear on the suppressor grid 90 substantially without change. Hence, the gating voltage of tube 76 will in effect be superimposed upon this varying cathode voltage, and thus will vary as a function of variations in the amplitude of the audio signal.

The division of current between the screen grid 80 and the anode 94 of pentode 76, at a constant suppressor grid voltage relative to the cathode, is a function of the current in the triode section of the tube. Thus vsome curvature is introduced into its cathode current-plate current characteristic which must be corrected if arnplitude distortion of the audio signal is to be avoided. This correction is brought about by intentionally predistorting to a selected degree the grid voltage-cathode current transfer characteristic of the tube. Since this intentional distortion is in an opposite sense to the cathode current-plate current curvature, the two types of non-linearity may be caused largely to cancel one an other, resulting in a compensated overall characteristic which has substantially less undesired curvature than the uncompensated characteristic alone. While certain details of this correction circuit will now be set forth, reference is made for a complete description to the copending application of E. M. Creamer,-Jr., Serial No. 70,952, filed January 14, 1949.

The circuit of Fig. 3a is designed as to include cathode y laora-aie I lz r degeneration and thereby to provide an amplitude-modulatedoutput in which substantially no width modulation of the pulses is present. As shown by the waveform 95 in Fig. 3b, the anode voltage of tube 76 remains -at a level of approximately 180 volts positive when no plate current is owing. The voltage on the screen grid 80 is approximately +75 volts. The two series resistors 86a and 86b act to hold the cathode 84 at a potential of Aapproximately 3 volts above ground ground asa result of the ow of screen current. Since the suppressor grid 90 is normally maintained at a potential of -55 volts, no plate current normally ows in the tube. However, when the suppressor grid 90 reaches a voltage of -10 volts above ground as shown in curve 96, current ows to lower the tube plate voltage as shown in curve 95. The cathode current is unchanged by this transfer of screen current to the plate.

A typical portion of the signal from the audio filter 22 is shown in curve 97, and is applied to the control grid 78. This control grid 78, the screen grid 80, and the cathode 84, act together as a triode to produce an audio voltage at the cathode which varies between +1 and +5 volts under peak input conditions. This voltage is conducted through the diode 88 to the suppressor grid 90 to swing the voltage on this grid between -53 and -57 volts, approximately.

The timing pulse 16 is also shown in Figure 3b. and

y is present at the junction 98 for a period of 8.33 microseconds in each cycle of operation. The positive voltage at this junction 98 drives the suppressor grid 92 from -55 volts to +3 volts (as shown in curve 96) inthe absence of an audio voltage, or to between' +1 and +5 n volts if an audio voltage is present. Because of the prestime at which conductance ceases.

ence of this audio voltage on the cathode 84, the control grid 78, and the suppressor grid 90 of the pentode, the time at which the pulse 16 causes conductance to commence in the anode circuit of the tube is definite and independent of the audio voltage. The same is true of the The amount of plate current which flows at the peak period of the pulse 16 depends upon the magnitude of the audio voltage, and is such as to cause a variation in the plate voltage of the tube from a nominal value of 178 volts to approximately 177 or 179 volts.

When the timing pulse 16 begins, it cuts oi the diode 88. This prevents the internal impedance of the pentode, and the impedance of its cathode resistor, from placing a load on the delay network 14.

While the resistor 89, which is returned to 150 volts, permits the discharge of condenser 92 when the cathode 84 of tube 76 swings in the negative direction after a positive excursion, nevertheless the time constant of this combination is so chosen that condenser 92 retains substantially all of its charge during the presence of the timing pulse 16. It is this charge, as varied at an audio rate through the diode 88, that forms the base, or pedestal, for the timing pulse 16.

Band-limiting network In Fig. 3c there is shown schematically a preferred type of filter which is adapted to perform the functions of the band-limiting network 34 in Fig. 1. 1t will be recalled that the network 34 has a frequency-response characteristic such as shown by the curve 36 (Fig. l), that is, substantially flat from D.C. out to approximately the cut-off point of the lter, after which-severe attenuation occurs. This feature is possessed by the bridged-T filter shown in Fig. 3c in which the im# pedances Z1 and Z3 are reciprocal networks with respectv to the resistance R0, so that, if Z1Z2 equals R03,'then the filter has both image impedances equal to R5 at .allfrequencies.

A practical circuit arrangementfor the vfilter of'Fg.l 3c, together with values for the various componentsjis' il'lusl trated in Fig. 3d. It will be noted that the lter is made 

